Common solid soaps are normally produced by a framing method or a milling method by using fatty acid soap as the base and by adding sugars or polyols such as sucrose, glycerin, sorbitol, and propylene glycol as necessary.
The fatty acid composition has a major effect on the properties of soap. If a saturated higher fatty acid (C18 stearic acid etc.) having a high number of carbon atoms is used, the solidification point and the hardness normally increase, and the adjustment of the shape of solid soap is easy. However, the solubility and the foaming property in cold water decrease, and they tend to decrease the cleansing power and the feeling in use. On the other hand, if a large amount of C12 lauric acid, C14 myristic acid, etc. (hereinafter referred to as “medium-chain fatty acid”) is used as the fatty acid, the solubility and the foaming property in cold water are largely improved. However, the solidification point and the hardness significantly decrease, and production suitability as solid soap and the shape-retaining property deteriorate. In particular, C14 myristic acid is excellent in the cleansing property and low-irritability, and it is desired to be used as the main component of solid soap. However, if the amount exceeds 50 mass % in the total fatty acid, the solidification point and the hardness tend to decrease notably. Thus, when myristic acid is used in a solid soap, the content of about 50% thereof has virtually been the limit.
Especially in the case of transparent soap, it is necessary to add a substantial amount of sugars or polyols to achieve transparency. Thus, the decrease in the solidification point is large, and the use of a large amount of medium-chain fatty acids tends to be difficult.
That is, the structural mechanism of letting transparent soap be transparent is considered that opaque soap fibrous microcrystals, which are optically discontinuous in size with respect to visible light, are mainly severed perpendicularly to the fiber axes by the addition of the above-described sugars and polyols and refined to the size of a wavelength of visible light or less; as a result, the soap becomes transparent. Therefore, the hardness and the solidification point easily decrease compared with the soap in which sugars and polyols are not added.
In particular, when transparent soap is produced by the framing method without using ethanol as the solvent for sugars and polyols, cutting, shape forming, and packaging are often carried out immediately after the removal of the frame. Thus, the decrease in the solidification point and the decrease in the hardness also directly lead to the deterioration of production suitability.
Therefore, the use of a large amount of myristic acid, which tends to lower the hardness and solidification point, has been difficult.
On the other hand, soaps in which amino acids or trimethylglycine is blended are publicly known (Japanese Unexamined Patent Publication No. 2001-40390 and WO2004/029190); however, the presence of adjustment effects for the decrease of the solidification point and the hardness, when a large amount of myristic acid is blended, has been totally unknown.